The present invention relates to a well tool for use in a down-hole drilling assembly. More particularly, the present invention relates to a well tool for centralizing a bottom-hole assembly in a down-hole drilling apparatus wherein the drill bit is rotated without drill pipe rotation.
In directional drilling operations where it is desired that the borehole be deviated or drilled off vertical, it is common to use down-hole drilling machines or apparatus in which the drill bit is rotated to effect the drilling operation without rotation of the drill pipe. For example, one type of such down-hole drilling apparatus employs a positive displacement, down-hole mud motor which rotates the drill bit and a rotatable drill bit sub. In a deviated hole, it is not uncommon that the bottom-hole assembly, i.e. the assembly nearest the down-hole mud motor, will stick to the wall of what is commonly referred to as the "low side" of the hole. It will be appreciated that in deep, deviated well bores, there is a large weight of drill pipe providing a large, vertically downward vector of force on the drill string. The situation can be particularly acute in cases where there is a large deviation off vertical, i.e. where the well bore is deviated to a near horizontal path. In such cases, since the drill string is not rotating, and depending upon the formation encountered, because of the downward force the drill string will stick on the low side of the dog leg or curvature adjoining the generally vertical part of the borehole with the generally horizontal tending part of the borehole. When such wall sticking occurs, drilling progress is essentially stopped since the drill string is prevented from any further movement axially along the well bore.
To overcome this wall sticking problem, it is common to condition the drilling mud to break the sticking or suction between the wall of the well bore formation and the drill string. Thus, gellants and other additives which act as lubricants are added to the mud in an attempt to decrease the friction between the well bore formation and the drill string and allow the latter to slide down the deviated hole. Other techniques involve the addition to the mud of solid materials such as walnut hulls, rice hulls, etc. which are pumped down the hole and which act as bearings so as to allow the bottom-hole assembly to slide in the deviated hole. These techniques are unsatisfactory as they are expensive and time consuming. Additionally, because the conditioning of the mud and freeing of the stuck drill string is so time consuming, it frequently happens that by the time the bottom-hole assembly has been freed, it is necessary to trip the string to replace the drill bit, meaning that the mud conditioning procedure may have to be repeated after a new bit has been attached to the drill string and sent downhole.